Mesoporous Materials
Materials whose structures allow fluids to flow through the materials are porous. Porous materials can be characterized by their pore sizes. Very small pores having diameters &lt;2 nm are called micropores, while very large pores (&gt;50 nm) are called macropores. Most high surface area carbons, such as activated carbons, are primarily microporous, and have pores that are too small to be readily accessible to liquids. Pores of intermediate size (between 2 and 50 nm) are called mesopores and form the subject of the present invention. One aspect of mesopores is that they have pores that are large enough to readily allow liquids to enter the material. At the same time, large pores do not provide as much surface area in a given volume of material as do smaller pores. Thus, mesoporous materials provide liquid access to more surface area per unit volume of material than either microporous or macroporous materials. As used herein the term "mesopore" will be used to refer to pores in the desired size range, namely to pores having diameters between approximately 2 and 50 nm.
Uses for Mesoporous Materials
Because of their large liquid-accessible surface areas, mesoporous materials are useful in many liquid phase applications, including as sorbents, electrical materials and catalyst supports. For example, catalytic reactions are typically surface reactions, i.e. the surface of the catalyst serves as an active site for the combination or separation of reactive species. The larger the surface area of catalyst, the more active sites there will be and the more rapidly the catalyzed reaction will proceed. Catalytic reactions can occur in the gas or liquid phase; in either case, it is desirable to maximize the amount of catalyst surface area. One common way to do this is to provide the catalyst as a thin coating on the surface of a support material. The support material provides the structure for the catalyst and thus determines its shape and the amount of surface area per unit volume. Hence, the porosity and surface area of the support become rate-limiting factors. Mesoporous materials provide optimal support structures for certain catalytic systems.
Similarly, pore size is an important aspect of sorbent technology. Sorption typically involves a solid phase (the sorbent) and a liquid or gas phase. The liquid phase can comprise a solvent containing a dissolved species to be sorbed, or an emulsion or other mixture of two liquids, one of which is to be sorbed. Examples of common sorbent applications currently include polymers and carbons in powdered, granular, and pelletized form for environmental applications associated with energy production, by-product recoveries, and waste incineration, as well as water purification and wastewater cleanup. An example of a sorbent application that involves a gas phase is air purification. A variety of other uses are known or are being developed. For each desired application, the sorbent is selected such that the sorbent has an affinity for the sorbate species, so that it is attracted into the solid and held there by one of various surface mechanisms. The efficacy of a sorbent material depends on how much sorbate it can attract and retain. Hence, the pore size and available surface area are critical in this context as well.
Similarly, certain electrical applications involve liquid phase, surface-limited reactions. One example of such an application is an ultracapacitor. Like batteries, ultracapacitors are energy storage devices. Ultracapacitors are notable for their ability to store and deliver energy at high power densities, and to be cycled virtually indefinitely without degradation. In contrast, batteries store large amounts of energy, but function most efficiently at low power densities and degrade quickly if they are deeply cycled. The characteristics of ultracapacitors make them particularly suitable to meet the power requirements of various emerging technologies, including electric vehicles, electronics (cellular telephones, and digital communications) and clean power (uninterrupted power sources, filters, etc.).
An ultracapacitor typically comprises a pair of electrodes separated by a non-conductive porous separator. The space between the electrodes is filled with a liquid electrolyte, which can be aqueous or organic-based. Because there are no chemical reactions taking place during the charge/discharge cycle, capacitors can be cycled many times without degradation, unlike batteries. However, previously known ultracapacitors lacked sufficient energy storage capacity to make them commercially practical. One key to improving the energy storage capacity of ultracapacitors is to optimize the interaction between the electrodes and the electrolyte.
There are two major categories of electrolytes for double layer devices: aqueous and organic, each of which has advantages and disadvantages. Aqueous electrolytes such as potassium hydroxide and sulfuric acid have low resistance (0.2 to 0.5 ohms/cm.sup.2) and can be charged and discharged very quickly. However, they can only be cycled through a potential range of one volt due to the voltage limits of aqueous electrolytes; this sharply limits their energy storage density (which is proportional to voltage squared). Organic electrolytes such as propylene carbonate have much higher breakdown voltages (up to three volts) and therefore have much greater energy storage densities (in theory, by a factor of nine). However, because they have much higher resistance (1-2 ohms/cm.sup.2), they cannot be cycled as quickly. The type of electrolyte that is desirable depends on the nature of the application.
The mechanism for energy storage devices of this type is based on the double-layer capacitance at a solid/solution interface. More specifically, double-layer ultracapacitors typically consist of high surface area carbon structures that store energy in a polarized liquid layer. The polarized liquid layer forms at the interface between an ionically conducting liquid electrolyte and an electronically conducting electrode, namely the carbon structure. As illustrated in FIG. 1, the separation of charge in the ionic species at the interface (called a double layer) produces a standing electric field. Thus, the capacitive layer, while only a few angstroms thick, has a very is large area. The larger the area of the interface is, the more energy can be stored. Hence, the capacitance of this type of capacitor is proportional to the surface area of the electrode.
At the same time, electrodes having pores smaller than about 2 nm do not exhibit increased capacitance. It is believed that pores smaller than about 2 nm are too small to allow entry of most nonaqueous electrolytes and therefore cannot be fully wetted, with the result that a portion of the potential interface area is not realized. Hence, it is believed that mesoporous materials are optimal for use in this type of capacitor.
While some carbon structures having pore sizes in the mesoporous range have been extensively investigated for use in ultracapacitors because of their low cost and potential for high-energy storage densities, none of them have proved entirely satisfactory. Since the capacitance of the material increases linearly with the specific surface area, a carbon material with a capacitance of 20 .mu.F/cm.sup.2 and a surface area of 1000 M.sup.2 /g would have a capacitance of 200 F/g if all of the surface were electrochemically accessible. However, since high surface area porous carbons typically have a high fraction of micropores, only a fraction of the surface of the carbon is effectively utilized (wetted); most of the surface therefore does not contribute to the double-layer capacitance of the electrode and the measured capacitance values of prior carbon structures are therefore only about 20% of theoretical. For carbon-based ultracapacitors to approach their theoretical performance, they should have a high pore volume (&gt;50%) and a high fraction of continuous pores with diameters of greater than 2 nm to allow the electrolyte access to the carbon surface.
In sum, the major drawbacks of the carbons now used in double-layer ultracapacitors are: low capacitance (due to pores that are too large or too small), low density (which increases the size of the ultracapacitor), and high costs (due to materials and processing costs).
In addition, the electrical conductivity low conductivity (due to resistance at particle/particle interfaces) of the electrode itself affects the efficiency of the capacitor. Thus, for ultracapacitor electrodes, monolithic carbon is more desirable than particulate carbons or compacts of particulate carbon, which have high surface areas, but which suffer from high internal resistance because of the particle-particle interfaces.
Another example where a monolithic polymer is advantageous is in high performance liquid chromatography (HPLC), a commonly used technique for separating and quantifying the constituents of a mixture. HPLC is often used to separate chemicals and biological molecules that have very similar properties and are difficult if not impossible to separate by other conventional means. The major disadvantage of HPLC is, because the columns are packed with small porous beads, the high flow rates required to maximize the throughput in preparatory scale separations result in channeling of the solution around the particles, thereby degrading the separation. Since separation is a major cost of chemical processing, the development of high capacity monolithic columns could greatly reduce the cost of manufacturing pharmaceuticals and their precursors. The mesoporous polymers described herein may be prepared by polymerization within a suitable structure (e.g., a glass or metal tube) to form a stationary phase for chromatography.
Manufacture of Mesoporous Materials
In order to introduce larger pores into polymers and carbon, and thus increase its porosity, several groups have tried to form polymeric gels around liquid emulsions. This usually results in a mixture of pore sizes, and both the polymers and the carbons formed therefrom are mostly macroporous (&gt;50 nm) rather than mesoporous (LeMay et al. 1990, Even and Gregory 1994). In an alternative approach, pyrolysis of aerogels prepared by supercritical fluid extraction of RF gels produces carbons with a mixture of meso- and micropores (Pekala et al. 1994), but because of the need for supercritical extraction, aerogels are very expensive to make. Hence, an effective method for producing a mesoporous carbon that does not involve supercritical extraction is particularly desirable.
On another front, since the invention of a new family of mesoporous silica materials, designated M41S, by scientists at Mobil Oil Corporation (Kresge et al. 1992, Beck et al. 1992), there have been numerous publications describing the use of surfactants to produce mesoporous metal oxides (Beck et al. 1994, Huo et al. 1994). This has dramatically expanded the range of pore sizes in metal oxides from the micropore to the mesopore regime. These mesoporous metal oxides are produced by gelation of metal alkoxides around a template made from micelles or liquid crystals formed by surfactants. Once the structure has formed, the surfactant is removed by high temperature oxidation, leaving a mesopore. The pore sizes can be adjusted by changing the length and the structure of the surfactants used. Cationic, anionic, and nonionic surfactants have been used to make a variety of mesoporous metal oxides (Luca et al. 1995).
Hence, it is desired to provide an improved mesoporous polymer and carbon structure. The desired polymer structure should be simple and inexpensive to manufacture and should have a high pore volume and a high fraction of mesopores. When intended for use as electrodes for ultracapacitors, the desired carbon structure should have high volumetric capacitance, high density, and high conductivity.